Iron Man #55 has been one of the hottest comic books over the last few years. It was always considered a key book because there are so many first appearances in it but it really exploded when Thanos became such a key character in the Marvel movies.
We still haven't seen more than a few cameos of Thanos in the Marvel movies but the infinity gems keep getting mentioned and Thanos is known for the whole Infinity Gauntlet storyline so you know he will be in the movies soon.
As for Iron Man #55 I finally purchased it for several hundred dollars which was much more than it cost back when I bought all my other Iron Man comics. Sometimes comics really do continue to go up in value but who would have thought this one would continue to climb like it has.